1. Field of the Invention
The present invention relates to an LPP (laser produced plasma) type EUV (extreme ultra violet) light source apparatus that generates extreme ultra violet light to be used for exposing a semiconductor wafer or the like.
2. Description of a Related Art
Recently, as semiconductor processes become finer, photolithography has been making rapid progress toward a higher resolution, and for the next generation, micro-fabrication of 100 nm to 700 nm, and further, micro-fabrication of 50 nm or less is being required. Accordingly, in order to meet the requirement of micro-fabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating extreme ultra violet light with a wavelength of approximately 13 nm and reduced projection reflective optics.
The EUV light sources include three kinds, namely, an LPP (laser produced plasma) light source using plasma generated by irradiating a target with a laser beam, a DPP (discharge produced plasma) light source using plasma generated by electric discharge, and an SR (synchrotron radiation) light source using orbital radiation light. Among these, the LPP light source is considered to be most promising as a light source for EUV lithography for which power of tens of watts or more is required. This is because the LLP light source has the advantages that an extremely high luminance close to black body radiation can be obtained because plasma density can be increased considerably, that light emission only in the necessary wavelength band is possible by selecting a target material, that an extremely large collection solid angle as large as 2π sterad can be ensured because of a point light source having almost an isotropic angle distribution and no structure around the light source such as an electrode, and so on.
FIG. 10 is a diagram showing an outline of a conventional LPP type EUV light source apparatus. As shown in FIG. 10, the EUV light source apparatus includes a driver laser 101, an EUV light generation chamber 102, a target material supply unit 103, and a laser beam collecting optics 104.
The driver laser 101 is an oscillation-amplification type laser apparatus that generates a driving laser beam to be used to excite a target material.
The EUV light generation chamber 102 is a chamber in which EUV light is generated and is evacuated by a vacuum pump 105 in order to facilitate turning the target material into plasma and prevent EUV light from being absorbed. In the EUV light generation chamber 102, a window 106 is attached, which causes a laser beam 120 generated by the driver laser 101 to pass through the inside of the EUV light generation chamber 102. Further, within the EUV light generation chamber 102, a target ejection nozzle 103a, a target collection tube 107, and an EUV light collector mirror 108 are arranged.
The target material supply unit 103 supplies a target material to be used to generate EUV light into the EUV light generation chamber 102 via the target ejection nozzle 103a, which is part of the target material supply unit 103. Among the supplied target materials, those not irradiated with a laser beam and no longer necessary are collected by the target collection tube 107.
The laser beam collecting optics 104 includes a mirror 104a that reflects the laser beam 120 output from the driver laser 101 toward the EUV light generation chamber 102, a mirror adjusting mechanism 104b that adjusts the position and angle (tilt angle) of the mirror 104a, a collecting device 104c that collects the laser beam 120 reflected by the mirror 104a, and a collecting device adjusting mechanism 104d that moves the collecting device 104c along the optical axis of the laser beam. The laser beam 120 collected by the laser beam collecting optics 104 passes through the window 106 and a hole formed in the center of the EUV light collector mirror 108 and reaches the orbit of the target material. In this manner, the laser beam collecting optics 104 collects the laser beam 120 so as to form its focus on the orbit of the target material. Due to this, the target material 109 is excited and turned into plasma and the EUV light is generated.
The EUV light collector mirror 108 is, for example, a concave mirror, on the surface of which a Mo/Si film that reflects light with a wavelength of 13.5 nm with a high reflectance is formed, and reflects generated EUV light 121 to thereby collect the light to IF (intermediate focusing point). The EUV light 121 reflected by the EUV light collector mirror 108 passes through a gate valve 110 provided in the EUV light generation chamber 102 and a filter 111 that removes unnecessary light (electromagnetic wave or light with a wavelength shorter than that of the EUV light, light with a wavelength longer than that of the EUV light, for example, ultra violet light, visible beam, infrared light, etc.) from among the light generated from the plasma and causes only the desired EUV light (for example, light with a wavelength of 13.5 nm) to be transmitted. The EUV light 121 collected to the IF (intermediate focusing point) is then guided to exposure equipment or the like via transmission optics.
Since a large amount of energy is radiated from the plasma generated in the EUV light generation chamber 102, the temperature of the parts in the EUV light generation chamber 102 is raised due to this radiation. Some techniques to prevent such a rise in temperature of parts are known.
As a related art, in Japanese Patent Application Publication JP-P2003-229298A, an X-ray generation apparatus is described, which comprises an X-ray source that turns a target material into plasma and radiates X-rays from the plasma, and a vacuum container that contains the X-ray source, and which is characterized in that an inner wall formed by a material having a high absorptance against the electromagnetic wave in the range from infrared to X-ray is provided on the inner side of the vacuum container. According to the X-ray generation apparatus, it is possible to prevent the parts within the vacuum container from being heated unnecessarily due to the radiation energy reflected and scattered by the inner wall of the vacuum container.
By the way, the plasma generated in the EUV light generation chamber 102 shown in FIG. 10 diffuses as time elapses and part of it scatters as atoms or ions. The inner wall and the structures of the EUV light generation chamber 102 are irradiated with the atoms or ions.
Due to the irradiation with the atoms scattered from the above-mentioned plasma, the following phenomenon may occur.
(1) The atoms scattered from the plasma stick to the surface of the window 106 at the inner side of the EUV light generation chamber 102. The atoms having thus stuck to the surface of the window 106 at the inner side of the EUV light generation chamber 102 absorb the laser beam 120.
Due to the irradiation with the ions scattered from the above-mentioned plasma, the following phenomena may occur.
(2) The surface of the window 106 at the inner side of the EUV light generation chamber 102 is irradiated with the ions scattered from the plasma and the surface of the window 106 at the inner side of the EUV light generation chamber 102 may deteriorate (the surface becomes coarse and unsmoothed). Due to this, the window 106 absorbs the laser beam 120 output from the driver laser 101.
(3) The inner wall and the structures of the EUV light generation chamber 102 are irradiated with the ions scattered from the plasma. Due to this sputtering, the atoms scattered from the inner wall and the structures of the EUV light generation chamber 102 stick to the surface of the window 106 at the inner side of the EUV light generation chamber 102. In this manner, the atoms having stuck to the surface of the window 106 at the inner side of the EUV light generation chamber 102 absorb the laser beam 120.
(4) Since the window 106 absorbs short-wavelength electromagnetic waves (light) generated from the plasma, the material thereof deteriorates. Due to this, the window 106 absorbs the laser 120.
If the above-mentioned phenomena (1) to (4) occur, the energy to turn the target material into plasma is lowered and the efficiency of generation of the EUV light 121 is decreased.
In addition, if the window 106 or the atoms having stuck to the window 106 absorb the laser beam 120, the temperature of the window 106 rises and distortion occurs in the window 106, and the ability to collect light decreases. Such a decrease in the ability to collect light causes a further decrease in the efficiency of generation of the EUV light 121. Furthermore, if the distortion of the window 106 becomes larger, it may eventually lead to damage of the window 106.
There may be a case where part of the laser beam collecting optics 104 (for example, lens, mirror, etc.) is arranged within the EUV light generation chamber 102. In such a case, also at the part of the laser beam collecting optics 104 arranged within the EUV light generation chamber 102, the phenomena in the above-mentioned (1) to (4) may occur. In particular, when a mirror that reflects the laser beam is arranged within the EUV light generation chamber 102, if the phenomena in the above-mentioned (1) to (4) occur, the reflectance of the laser beam of the enhanced reflection coating of the mirror reflecting surface decreases. Due to this, the energy to turn the target material into plasma is lowered and the efficiency of generation of the EUV light 121 decreases.
In general, in the field of optics, it is known that the shorter the focal length, the smaller the image is, and the longer the focal length, the larger the image is. Taking this into account, it is preferable to reduce the light collection size (spot size) of the laser beam 120 by reducing the focal length of the laser beam collecting optics 104 in order to improve the efficiency of generation of the EUV light 121. However, in order to reduce the focal length of the laser beam collecting optics 104, it is necessary to reduce the distance between the window 106 and the plasma. Because of this, it becomes more likely that the phenomena in (1) to (4) described above occur on the surface of the window 106 at the inner side of the EUV light generation chamber 102.
In addition, as mentioned above, in order to increase the transmittance of the EUV light 121 generated from the plasma, it is necessary to maintain the inside of the EUV light generation chamber 102 at substantially vacuum by the vacuum pump 105. Because of this, the heat at the surface of the window 106 at the inner side of the EUV light generation chamber 102 or at part of the laser beam collecting optics 104 arranged inside the EUV light generation chamber 102 is difficult to diffuse and the deterioration of the devices will proceed.